Monitoring of friction stirred spot weld quality for dissimilar Al6061 to polycarbonate using tool assisted thrust-torque signatures

Abstract

Friction stir processing procedures are often found to be an alternative choice for the joining of thermoplastic polymers without melting the base materials. In recent years, dissimilar metallic to thermoplastic assemblies are highly used in aerospace and automotive industries as mechanical fastener or fusion welding techniques are rather inadequate for extreme discrepancy in thermal properties. The present work addresses the feasibility of friction stirred spot welding for non-ferrous aluminium (Al 6061) to polycarbonate using taper cylindrical tool of hardened H13 steel along with machine integrated axial thrust and stirring torque signals. The real-time positional and axial velocity of the rotating tool has been acquired to distinguish between plunging and dwelling stage. The process stability that is, the deviation of thrust-torque in respective phases was processed. The interaction of plunge depth and dwell time on tool pin assisted keyhole with associated material expulsion for different tool revolving speed was established. The weld mechanical behaviour has been scanned using load versus elongation diagram with associated weld pair interface. Lastly, sensors’ based process monitoring extended to weld quality prediction have been made using various significant features of thrust-torque-velocity based approaches. The tool spinning speed followed by dwell time was found to be important on weld quality index. The joint efficiency was significantly upgraded (more than 60%) at intermediate dwell time (40 s) with low tool plunging (0.3 mm) or reversed parametric setting at high tool revolving speed (2500 rpm) due to substantial reduction in axial thrust with higher stirring torque. The local deviation in thrust-torque during plunging was equally responsible with global mean thrust-torque in dwelling to dictate the joint strength and the weld ductility. The material stirring torque was better indicator of weld shear strength while pin assisted slot size highly correlated with tool thrust though tool plunging speed was highly crucial.